The preheating provides for localized achievement of a higher temperature than the ambient conditions, from both sides of the joint and for a definite width.

Generally, the manufacturing standards regulate the requiring of the treatment; the process is then validated in the welding procedure qualification.

The basic reasons of carrying out a preheating are:

  • Reducing the cooling gradient, especially below 800 °C, in order to limit the hardness values, especially in the Heat Affected Zone;
  • Controlling the hydrogen diffusion coefficient, and therefore acting against the risk of cold cracking;
  • Limiting the welding residual stress and the risk of cracking due to the constraint state;
  • Offsetting the high thermal conduction coefficients of some alloys.


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